Power conversion, from alternating current (AC) to direct current (DC) and/or from DC to AC is achieved today predominantly by voltage source converters (inverters and rectifiers) having a switching circuit. For example, for a rectifier, either a three phase or single phase AC voltage source supply is rectified to a controlled or uncontrolled DC bus voltage. Inverters in either three phase or single phase power circuits use a fixed DC bus voltage that may be center-tapped to create a modulated stepped waveform on the AC side using a switching algorithm. The stepped waveform is filtered via an AC filter to create an AC voltage. The AC voltage is usually regulated by altering the switching pattern, or by regulating the DC bus voltage, or a combination thereof. The resulting waveform can be connected to a dead load (resistor, inductor, and capacitor) or paralleled with another voltage source, depending on the application.
Typical applications include rectifiers and high current rectifiers (for DC loads), frequency converters, UPSs, solar inverters, battery energy storage systems, etc. The power conversion described above is achieved using voltage source technology. The same power conversion objectives can be achieved using current source technology. This paper seeks to explain a method for the generation of a true current source and how it can be applied in the domain of AC/DC and DC/AC power conversion.
FIG. 1A is a schematic diagram illustrating a series connected inductor and capacitor, or an LC circuit to transform an AC voltage source to an AC current source at the load terminals, load 207. Referring to FIG. 1A, circuit 200 transforms an AC voltage source into an AC current source at load 207. AC voltage source 201, such as an AC 60 Hz 120V, power source provides a constant voltage to circuit 200. Tuning inductor 203 and capacitor 205, having a resonant frequency of 1/(2*pi*SQRT(LC)), to the voltage source frequency, e.g., 60 Hz, as the load resistance tends toward infinity the load voltage also tends toward infinity. An infinite electro motive force (EMF) or voltage and infinite resistance defines a current source. Here, L is inductance of inductor 203 and C is capacitance of capacitor 205.
FIG. 1B is a schematic diagram illustrating a series connected inductor with a parallel connected capacitor and inductor, or a resonant LCL circuit to transform an AC voltage source to an AC current source at the load terminals. Referring to FIG. 1B, circuit 250 transforms an AC voltage source into an AC current source at load 259. AC voltage source 251, such as an AC 60 Hz 120V, power source provides a constant voltage to circuit 250. Tuning inductor 253 and capacitor 255, having a resonant frequency of inverse of 2π multiplied by the square root of (LC), to the voltage source frequency, e.g., 60 Hz, drives the circuit to behave with a constant current characteristic. Here, L is inductance of inductor 253 and C is capacitance of capacitor 255. Inductor 257, also tuned to the supply frequency, connected in series with load 259 ensures no current is drawn from the supply at no load due to parallel resonance. Parallel resonance occurs when a circuit current is in phase with the applied voltage of an AC circuit containing an inductor and a capacitor connected in parallel. The circuit creates a true AC current source at load 259, e.g., a current in resistance 259 that is independent of the magnitude of the resistance of load 259.
Appendix A shows calculations for a general case of a 1200 W AC current source of FIG. 1B. The equations prove that if the reactors and capacitors are designed for equal power and tuned to the supply resonant frequency the output current does not vary with a change of load, from zero to nominal ohms for component 259.